73 Diamonds-Mid

Diamonds FOCUS Question How do diamonds form?

What Is a Diamond? Have you ever seen a diamond? It is a beautiful gemstone. Not all diamonds are clear and sparkly, however. Some are dull, They can be brown, gray, or black. They can even be white, yellow, or pink. No matter what color, all diamonds are minerals made of the element carbon. Like many other minerals, diamonds are crystals. Crystals are organized groups of atoms. The shape of a crystal depends on how the atoms are arranged. Most crystals have straight edges and smooth sides. Each side is called a face.

Getting Into Shape Diamond crystals usually have eight faces. Their crystals are octahedrons (ock-tuh-HE-druns). They look like two four-sided pyramids, stuck together bottom to bottom. X-ray pictures help us see how diamonds get their shape. The diagram above shows carbon atoms in a diamond crystal. Compare it to the shape of the diamond. The pattern of atoms in one crystal is the same as the shape of the whole diamond.

A Tale of Two Carbons Diamonds are not the only minerals that are made of carbon. Graphite is also made of pure carbon. Why are diamonds and graphite so different? A bond attaches one atom to another atom. Atoms can bond to each other in different ways. Some bonds are weak. Some are strong. The way a crystal forms affects the strength of bonds that attach atoms together. Diamonds form under very high temperatures and pressure. Diamonds have very strong bonds.

Graphite forms under much lower temperatures and pressure than diamonds do. The bonds that attach the carbon atoms in graphite crystals are much weaker. This makes graphite soft and slippery. These properties make graphite good for use in pencils. Science in Your World The "lead" in your pencil is actually graphite. Graphite has been used for writing since the 16th century. The Englishmen who began using it thought graphite was lead.

Super Crystals! Earth's outer layer is the crust. Under it is the mantle. Diamonds form in the mantle, about 160 kilometers (100 mi.) below Earth's surface There, rock is under a lot of pressure because of the heavy rock above it It is also very hot. It is so hot in Earth's mantle that some of the rock melts and turns into magma. Diamonds form as magma cools and turns back into solid rock. If the magma cools slowly, the diamond crystal grows bigger. This happens because there is more time for carbon atoms to bond together. If the magma cools quickly, crystals do not have as much time to form The diamonds are smaller.